The present invention relates generally to a structure of a multi-domain wide-viewing angle liquid crystal display (MD-WVA-LCD), and more specifically to a structure of a multi-domain wide-viewing angle-liquid crystal display with a superimposed spacing wall-bump structure.
The market for liquid crystal display (LCD) panels is increasing rapidly, especially in their applications to notebook PCs and monitors. When LCD panels of larger size and higher resolution are used for desktop monitors, wide-viewing angles (WVA) and fast response time become very critical in meeting the monitors requirements. In addition to contrast ratio with respect to different viewing angles, gray-scale inversion, colorimetry, and the optical response of an LCD are important features of a high quality LCD panels. However, the cost associated with designing and manufacturing a panel also needs to be considered.
Controlling liquid crystal domains is the most important technology in obtaining a wide-viewing angle for an LCD. In the technology for flat panel display, each pixel is divided into several domains to compensate for the asymmetry in optics so as to increase the viewing angle of an LCD panel. Most of the conventional LCDs are 90xc2x0 twisted nematic (TN) LCDs having an LCD panel and crossed polarizers attached outside. The drawbacks of the conventional LCDs include narrow viewing angles (xc2x140xc2x0 horizontally and xc2x130xc2x0 vertically), slow response (about 50ms), and large color dispersion. Therefore, it is difficult to make high quality LCD panels. Also, the black matrix of the twisted nematic LCD must cover the disclination lines generated by reverse-twisted and reverse-disclined liquid crystal molecules. In addition, the rubbing process required in manufacturing the panels causes electric static damage (ESD) and particle pollution.
Because of the demand in high quality LCD panels with wide-viewing angles, the structure of vertically aligned multi-domain LCDs has been developed. For instance, Fujitsu""s multi-domain vertically aligned (MVA) structure has W-shaped bump structure as shown in FIG. 1. The W-shaped bump structure is formed on the upper and lower substrates to form a multi-domain LCD cell of multiple independent directions. Its LCD panel has a wider viewing angle and smaller color dispersion than conventional LCD Ad panels but only about 55% light intensity of a conventional TN LCD. Also it may generate reverse disclination lines because the angle between its bump and its pixel electrode is formed by 45xc2x0. Moreover, the horizontal gap between the upper and the lower bumps must be less than 30 xcexcm. It needs high precision to align. Therefore, the design specification is not easy and the process window is smaller.
U.S. Pat. No. 5,907,380 also discloses a structure of liquid crystal molecules that uses thin walls to provide a lateral fringe field to control the tilt direction of the liquid crystal molecules. This thin wall structure is formed on a pixel electrode layer of a single substrate. This LCD needs higher driving voltage and the response time is longer.
In addition, H. D. Liu et al. disclose a structure of a multi-domain homeotropic aligned LCD in xe2x80x9cA Wide Viewing Angle Back-Side Exposure MVA-TFT-LCD with Novel Structure and Simple Processxe2x80x9d, The 19th International Display Research Conference, pp. 31-34, 1999. In the art, it utilizes a back side exposure (BSE) method to manufacture a surrounding wall-bump (SWB) structure. The wall-bump structure is formed around the pixel electrode on a single substrate having a thin film transistor (TFT). It also uses the combination effect of a fringe field and a bump structure to form multi-domains of the liquid crystal molecules and to increase the optical transmittance for the LCD, as shown in FIG. 2. The wall-bump 202 around the pixel electrode 201 is formed on the bus line of a thin film transistor substrate 203. The wall-bump around the pixel electrode has many advantages for an LCD. Neither rubbing process nor polymer stabilized ultraviolet curing process is needed in manufacturing the LCD. However, the LCD only has 60% light intensity of a conventional TN LCD.
All of the manufacturing processes of the aforementioned liquid crystal displays need to fill spacers between the upper and lower substrates to form the cell gap of liquid crystals. The uniformity of the cell gap is not easy to control. They also have light leakage problems, especially for a TN LCD under the operating mode of being normally white.
The present invention overcomes the difficulties and the disadvantages of the structure of conventional multi-domain liquid crystal displays. The primary object of the present invention is to provide a structure of a multi-domain wide-viewing angle liquid crystal display with a superimposed spacing wall-bump (SSWB) structure. The superimposed spacing wall-bump structure is formed on both upper and lower parallel substrates.
According to the invention, the SSWB structure formed on the upper and lower substrates is easy to design for the application in the larger pixel size and has larger process window. It also provides high pre-tilted angles for liquid crystal molecules and results in orderly alignment of the liquid crystal molecules to form multi-domain textures when an external voltage is applied.
In the preferred embodiments of a multi-domain wide-viewing angle liquid crystal display of the invention, the upper and lower substrates are parallel to each other. One of them has a color filter thereon and the other has thin film transistors thereon. Superimposed spacing wall-bumps are located at the pixel central portions on the color filter substrate and around the pixels on the TFT substrate. The SSWB at the central portion of a pixel on the color filter substrate is used to freeze the zone of the reverse disclination lines and the boundary of liquid crystal domains. It can also reduce the size of the zone of the disclination lines.
Another object of the present invention is to provide a manufacturing method for the aforementioned SSWB. The manufacturing method for the SSWB requires neither rubbing process nor polymer stabilized ultraviolet curing process. Also it does not need to fill spacers between the upper and lower substrates. It uses patterns of a photo mask to form uniform cell gap of liquid crystals. This manufacturing process is very simple and does not cause the light leakage problem. Therefore, it can increase the contrast ratio between the bright state and the dark state at various locations of the LCD.
The LCD of the present invention also increases the optical transmittance of the liquid crystal molecules. It has more than 85% light intensity of a conventional TN LCD and 20% higher optical transmittance than a conventional vertically aligned wide-viewing angle LCD.
The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.